As a transformer (voltage transformer), there is well known an electromagnetic transformer comprising windings wound around an iron core. The electromagnetic transformer is unsuitable for use in a power supply for a small-sized electric apparatus because it is bulky in size, is large in power consumption, and generates electromagnetic noise and heat. For example, for use in a high-voltage power supply in an electrostatic generating device or a back-lighting lamp of a liquid-crystal display, the transformer does not require a large output current but requires an output voltage between 1 kV and about several watts. In addition, it is required to reduce the electromagnetic noise, the power consumption, and the size.
On the other hand, since a piezoelectric transformer utilizing a piezoelectric phenomenon generates little electromagnetic noise and can be reduced in size, practical use is considered as a power supply transformer for a small-sized apparatus.
Referring to FIGS. 1(a) and (b), a conventional piezoelectric transformer 11 comprises a piezoelectric-ceramics rectangular plate 13, two surface electrodes 15 and 15 formed on the piezoelectric-ceramics rectangular plate 13 opposite to each other in a thickness direction at a part (hereinafter referred to as a first part) extending from one end to an approximate half in a longitudinal direction, and a plurality of internal electrodes 16 and 17 formed in the interior of the above-mentioned first part between the both surface electrodes with a space kept from one another in the thickness direction. Side electrodes 18 and 19 formed on confronting side surfaces of the above-mentioned first part, respectively, are connected to the surface electrodes 15 and 15, respectively, and to the alternate internal electrodes 16 and the remaining internal electrodes 17, respectively. Moreover, an end electrode 20 for output extraction is formed on the piezoelectric-ceramics rectangular plate 13 over an end surface of a half part (hereinafter referred to as a second part) opposite to the above-mentioned first part.
The above-mentioned first part of the piezoelectric-ceramics rectangular plate 13 is polarized by applying a DC voltage between the side electrodes 18 and 19. Specifically, the piezoelectric-ceramics rectangular plate 13 is polarized between adjacent electrodes of the surface electrodes 15 and 15 and the internal electrodes 16 and 17. The polarization directions are opposite to each other at both sides of each of the internal electrodes 16 and 17, as depicted by small arrows in FIG. 2(b). Furthermore, by applying a DC voltage between the both surface electrodes 15 and the end electrode 20, the second part of the piezoelectric-ceramics rectangular plate 13 is polarized in the longitudinal direction, as depicted by a large arrow in FIG. 1(b).
The above-mentioned type including the plurality of internal electrodes will be referred to as a stacked type because it is actually formed by alternately stacking the internal electrodes and piezoelectric members in manufacture. On the other hand, another type is also known in which the polarization in the thickness direction is only one direction between the confronting surface electrodes 15 and 15 without any internal electrodes formed. This type will be referred to as a single plate type because no stacking is required during manufacture and it is implemented by a single piezoelectric member with electrodes formed on its surfaces.
Description will be made as regards an operation of the piezoelectric transformer illustrated in FIGS. 1(a) and (b).
Now, one of the side electrodes 18 and 19 is used as an ground terminal and the other is applied as an input voltage with an AC voltage having a frequency equal to a resonant frequency of the piezoelectric-ceramics rectangular plate 13 in a one-wavelength resonance mode of a longitudinal vibration. Then, the stacked-type piezoelectric transformer acts as a piezoelectric vibrator to vibrate with a displacement distribution and a strain distribution illustrated in FIGS. 2(a) and (b), respectively. At this time, an AC voltage is produced between each of the surface and the internal electrodes 15, 16 and 17 and the end electrode 20 due to the piezoelectric effect. The level of the voltage thus produced is generally determined by distances between adjacent ones of the surface electrodes 15 and the internal electrodes 16 and 17, a distance between the surface electrodes 15 and the end electrode 20, and the input voltage.
Specifically, in the piezoelectric transformer, a transformed voltage can be obtained by energy conversion utilizing the piezoelectric effect, that is, electric-mechanical-electric conversion. In the meanwhile, in the piezoelectric transformer having the above-mentioned transforming system, the thickness of the piezoelectric plate is reduced and/or the thickness between the input electrodes (the surface electrodes and the internal electrodes) is reduced in order to satisfy the demands for low voltage driving, reduction in size, and a large step-up ratio (output voltage/input voltage). As a result, an input impedance is decreased so that an input current (motional current) is increased. The input current is converted into a vibration rate by mechanical conversion. Thus, in case where the vibration rate and an amplitude exceed a vibration level limit (this means the vibration rate at which the temperature (.DELTA.T) of the vibrator reaches a predetermined level due to heat generation at a high vibration rate and a large amplitude; the predetermined level can be selected, for example, as .DELTA.T=25.degree. C.) inherent to the used piezoelectric ceramics by the increase of the input current, there arises a disadvantage that the heat generation increases and an efficiency is decreased.
Therefore, in order to solve the above-mentioned drawback in the prior art, it is a technical object of this invention to provide a structure of a piezoelectric transformer which satisfies the demands for low power consumption, low voltage driving, reduction in size, and a large step-up ratio and which is low in vibration rate, small in heat generation, and high in efficiency.